Nano-imprint lithography was initiated as an alternative to conventional lithography techniques to achieve nanoscale structures with high throughput and low cost. The nanoscale structures are transferred from a mold to a polymer film covering a substrate during the imprinting process; see, e.g., U.S. Pat. No. 6,294,450, entitled “Nanoscale Patterning for the Formation of Extensive Wires”, issued to Yong Chen et al on Sep. 25, 2001; U.S. Pat. No. 6,407,443, entitled “Nanoscale Patterning for the Formation of Extensive Wires”, issued to Yong Chen et al on Jun. 18, 2002; and U.S. Pat. No. 6,432,740, entitled “Fabrication of Molecular Electronic Circuit by Imprinting”, issued to Yong Chen on Aug. 13, 2002, all assigned to the same assignee as the present application, the contents of which are totally incorporated herein by reference.
To deform the shape of the polymer, the substrate and polymer film are heated to above the glass transition temperature (Tg) of the polymer and the mold is pressed into the film, resulting in a reverse replica of the mold on the substrate. Poly(methyl methacrylate) (PMMA) has been a popular thermoplastic polymer despite the requirements for high operating temperature (˜200° C.) and pressure (˜2,000 psig).
For the PMMA process, a polymer solution (polymer dissolved in solvent) is spin-coated onto the substrate and then the polymer film is baked to remove any residual solvent inside the film. This baking procedure results in a hard polymer film, which in turn requires a high imprinting temperature, typically at least 50° C. above the Tg, and high pressure to ensure viscous flow of the polymer into the grooves and crevices of the mold before it freezes. The high pressure is detrimental to the nanoscale features on the stamp and both the high temperature and the high pressure can damage the functional materials of the circuits being fabricated, such as molecules or nanoscale semiconductor components, exposed to this process. The unwanted residual solvent in the polymer film results in film shrinkage and possible cracking associated with the solvent evaporation during the heating process, which can cause serious problems in pattern accuracy.
Recently, room-temperature imprinting lithography was suggested by vapor treatment of the dried polymer film (case 1) and by using new material, spin-on-glass (SOG; case 2). However, these processes suffer from the poor adhesion between polymer and substrate after solvent treatment (case 1) and fast drying of the spun film which also has a poor adhesion to metal due to its solvent nature (case 2).
There remains a need for an imprinting process that operates at a lower temperature and lower pressure than the prior art, as well as avoiding most, if not all, of the problems of the prior art.